1. Field of the Invention
Exemplary embodiments of the present invention relate to a power controlled adaptive modulation and coding (hereafter, referred to as ‘AMC’) scheme in a satellite communications system; and, more particularly, to a power controlled AMC scheme in a satellite communications system which has a long round trip time (RTT) because of its wide cell region.
2. Description of Related Art
The AMC scheme is a transmission scheme which has been used in 1×Evolved Data Only (1×EV DO) of a third generation (3G) mobile communication system standardized with the 3G Partnership Project 2 (3GPP2), and is being currently used in a variety of mobile communication systems including High Speed Downlink Packet Access (HSDPA), WiBro (WiMax), Digital Video Broadcasting—Satellite—Second Generation (DVB-S2), and 3GPP Long Term Evolution (LTE).
The 1×EV DO of the 3GPP2 is representative technology designed for packet services. It adopts the packet switching to improve the spectrum efficiency, which is disclosed in a paper published by Paul Bender et al., CDMA/HDR: A Bandwidth-Efficient High-Speed Wireless Data Service for Nomadic Users, IEEE communication Magazine, July 2000. In the forward link, packets transmitted to each user are multiplexed by each time slot in time division multiplexing. The packets in each time slot are always transmitted from a base station in a maximum available power.
Accordingly, each user measures a received signal to interference ratio (SIR) with respect to pilot symbols which are added to the central part of each time slot, selects a transmission rate appropriate for the current condition based on a predetermined transmission rate table, and reports the selected transmission rate to the base station. The base station performs scheduling for packets to be transmitted based on the transmission rate selected by each user, and transmits packets in a selected slot at the selected transmission rate. The 1×EV DO of the 3GPP2 supports 12 different packet transmission rates depending on different symbol repetition numbers, modulation types, and code rates. Since packets are transmitted at a maximum rate in the 1×EvDO of the 3GPP2, the 1×EvDO system has an advantage that it needs not perform power control in the forward link.
Basically, the AMC scheme is applied in the same manner as the 1×EV DO system of the 3GPP2. Depending on systems, a code assignment or subcarrier allocation scheme may be combined with the AMC scheme.
However, such an AMC scheme is designed under an assumption that the channel state is maintained within a constant range during a short RTT. Therefore, a terminal should continuously report information about the channel state to the base station at constant intervals. However, since satellite communications systems have a long RTT, they require a much longer time than ground communications systems, until the information reported at constant intervals is reflected. Therefore, it is difficult to apply the basic assumption of the AMC scheme that the channel state is maintained within a constant range.
Meanwhile, an AMC scheme such as DVS-2, which is used in a state in which the mobility of terminals does not exist, offsets the effects of rain attenuation. In this AMC scheme, channel variation is not fast in comparison with the long RTT of the satellite communications system. Therefore, there is no problem in applying the assumption. In the case of mobile communication using a satellite, however, channel variation caused by the long RTT is so severe that the same AMC scheme as the DVS-2 cannot be applied.